Power converter with isolated and regulation stages

ABSTRACT

In a power converter, the duty cycle of a primary winding circuit causes near continuous flow of power through the primary and secondary winding circuits during normal operation. By providing no regulation during normal operation, a very efficient circuit is obtained with a synchronous rectifier in the secondary operating at all times. However, during certain conditions such as start up or a short-circuit, the duty cycle of the primary may be reduced to cause freewheeling periods. A normally non-regulating isolation stage may be followed by plural non-isolating regulation stages. To simplify the gate drive, the synchronous rectifiers may be allowed to turn off for a portion of the cycle when the duty cycle is reduced. A filter inductance of the secondary winding circuit is sufficient to minimize ripple during normal operation, but allows large ripple when the duty cycle is reduced. By accepting large ripple during other than normal operation, a smaller filter inductance can be used.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/407,699, filed Apr. 20, 2006, issuing as U.S. Pat. No. 7,272,021 onSep. 18, 2007, which is a Continuation-in-Part of U.S. application Ser.No. 10/729,430, filed on Dec. 5, 2003, now U.S. Pat. No. 7,050,309,which claims the benefit of U.S. Provisional Application No. 60/431,673,filed Dec. 6, 2002 and a Continuation-in-Part to U.S. application Ser.No. 10/812,314, filed Mar. 29, 2004, now U.S. Pat. No. 7,072,190, whichis a continuation of application Ser. No. 10/359,457, filed Feb. 5,2003, now U.S. Pat. No. 6,731,520, which is a continuation ofapplication Ser. No. 09/821,655, filed Mar. 29, 2001, now U.S. Pat. No.6,594,159, which is a divisional of application Ser. No. 09/417,867,filed Oct. 13, 1999, now U.S. Pat. No. 6,222,742, which is a divisionalof Ser. No. 09/012,475, filed Jan. 23, 1998, now U.S. Pat. No.5,999,417, which claims the benefit of U.S. Provisional Application60/036,245 filed Jan. 24, 1997. The entire teachings of the aboveapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention pertains to switching power converters. A specificexample of a power converter is a DC-DC power supply that draws 100watts of power from a 48 volt DC source and converts it to a 5 volt DCoutput to drive logic circuitry. The nominal values and ranges of theinput and output voltages, as well as the maximum power handlingcapability of the converter, depend on the application.

It is common today for switching power supplies to have a switchingfrequency of 100 kHz or higher. Such a high switching frequency permitsthe capacitors, inductors, and transformers in the converter to bephysically small. The reduction in the overall volume of the converterthat results is desirable to the users of such supplies.

Another important attribute of a power supply is its efficiency. Thehigher the efficiency, the less heat that is dissipated within thesupply, and the less design effort, volume, weight, and cost that mustbe devoted to remove this heat. A higher efficiency is therefore alsodesirable to the users of these supplies.

A significant fraction of the energy dissipated in a power supply is dueto the on-state (or conduction) loss of the diodes used, particularly ifthe load and/or source voltages are low (e.g. 3.3, 5, or 12 volts). Inorder to reduce this conduction loss, the diodes are sometimes replacedwith transistors whose on-state voltages are much smaller. Thesetransistors, called synchronous rectifiers, are typically power MOSFETsfor converters switching in the 100 kHz and higher range.

The use of transistors as synchronous rectifiers in high switchingfrequency converters presents several technical challenges. One is theneed to provide properly timed drives to the control terminals of thesetransistors. This task is made more complicated when the converterprovides electrical isolation between its input and output because thesynchronous rectifier drives are then isolated from the drives of themain, primary side transistors. Another challenge is the need tominimize losses during the switch transitions of the synchronousrectifiers. An important portion of these switching losses is due to theneed to charge and discharge the parasitic capacitances of thetransistors, the parasitic inductances of interconnections, and theleakage inductance of transformer windings.

SUMMARY OF THE INVENTION

In certain embodiments of the invention, a power converter systemcomprises a normally non-regulating isolation stage and a plurality ofnon-isolating regulation stages, each receiving the output of theisolation stage and regulating a regulation stage output. Thenon-regulating isolation stage may comprise a primary winding circuitand a secondary winding circuit coupled to the primary winding circuit.The secondary winding circuit comprises a secondary transformer windingin series with a controlled rectifier having a parallel uncontrolledrectifier. A control circuit controls duty cycle of the primary windingcircuit, the duty cycle causing substantially uninterrupted control ofpower through the primary and secondary winding circuits during normaloperation.

The duty cycle of the primary winding circuit may be reduced to causefreewheeling periods in other than normal operation. Duty cycle might bereduced during the startup or to limit current and may be a function ofsensed current.

The primary winding circuit may include a single primary winding, andthe secondary winding circuit may include plural secondary windingscoupled to the single primary winding. The primary winding may be in afull bridge circuit having a capacitor in series with the primarywinding. In one implementation of the full bridge circuit, duringfreewheeling, only two top FETs or two bottom FETs are turned off.

A control signal of the controlled rectifier may be derived from awaveform of the secondary winding circuit. The secondary winding circuitmay include a filter inductor and have a capacitor coupled across itsoutput.

The isolation stage may be a step down stage. For example, it mayprovide an output of about 12 volts from a DC power source that providesa voltage varying over the range of 36-75 volts. The regulation stagesmay be down converters to provide outputs of voltage levels to drivelogic circuitry. A regulation stage output may, for example, be 5 voltsor less, such as 3.3 volts.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 shows a full-bridge, single-transformer, voltage-fed isolationstage that incorporates concepts of the '417 patent.

FIG. 2 illustrates the addition of a capacitor to the primary winding ofFIG. 1.

FIG. 3 illustrates the addition of an output filter inductor to thecircuit of FIG. 2.

FIGS. 4A-4C show a control circuit for the circuits of FIGS. 1-3 andembodying the present invention, and FIG. 4D shows an alternative to thecircuit of FIG. 4B.

FIG. 5 shows an Intermediate Bus Architecture (IBA) implementation ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 shows a full-bridge, single-transformer, voltage-fed isolationstage that incorporates synchronous rectification and the concepts ofthe '417 patent. The operation of this isolation stage is as follows.For the first half of the cycle, MOSFETs 101 and 103 are turned on whileMOSFETs 102 and 104 are left off, and the voltage VB is appliedpositively (according to the “dot” convention) across the transformer'sprimary winding 107. This voltage, modified by the transformer'sturns-ratio, appears across the secondary windings with the appropriatepolarity. Power flows into the transformer's primary winding, and out ofthe first secondary winding 108 to the output. The voltage at Node B isapproximately twice the output voltage, and it causes the MOSFETsynchronous rectifier 105 to be turned on. The voltage at Node A istherefore slightly below ground, which causes the MOSFET synchronousrectifier 106 to be turned off. These states of the rectifier switchesare consistent with the power flowing out of the first secondarywinding.

During the second half of the cycle, MOSFETs 102 and 104 are turned onwhile MOSFETs 101 and 103 are left off, and the voltage VB is appliednegatively across the transformer's primary winding. This negativepolarity causes MOSFET 106 to be turned on, MOSFET 105 to be turned off,and power to flow into the primary winding and out of the secondsecondary winding 109 to the output across capacitor 110.

The secondary windings are not tightly coupled to each other, asindicated with the parasitic inductances 113 and 114, to achieve theadvantages discussed in the '417 patent. A similar setup was shown inthe topology of FIG. 9 of the '417 patent since it also used a singletransformer.

Care must be taken in this isolation stage topology to insure that themagnetizing inductance of the transformer does not saturate. One way todo this is to place a large capacitor 215 in series with the primarywinding, as shown in FIG. 2. This capacitor will assume a dc voltageacross it that counters any imbalance there may be in the positive andnegative volt-seconds of the waveforms created by MOSFETs 101-104.Alternatively, several well-known techniques to sense the magnetizinginductor's current could be used to modify the durations of the firstand second halves of the cycle.

The filters at the output of the isolation stages in the '417 patent arecomposed of one or more capacitive and inductive elements. When theisolation stage is voltage-fed, it may be desirable to have the outputfilter begin with an inductor 316, as shown in FIG. 3. One benefit thisapproach provides is that the voltage-fed isolation stages can now beoperated with a variable duty cycle control strategy to provide asoft-start capability or to limit current flow in a short-circuitcondition. These functions could be provided by the regulation stages inthe topologies depicted in the '417 patent, but if the isolation stageis not combined directly with a regulation stage in a single product,then it may be desirable to include these functional capabilities in theisolation stage, as well.

Under variable duty cycle control, the percentage of the overall cycle(the duty cycle) that MOSFETs 101 and 103 (or MOSFETs 102 and 104)conduct is reduced from the 50% value described above. For theremaining, freewheeling fraction of the half-cycle, either all of theprimary-side MOSFETs are turned off, or at least the two top MOSFETs 101and 104 or the two bottom MOSFETs 102 and 103 are turned off. During thefreewheeling part of the cycle, both diodes 111 and 112 conduct thecurrent flowing through inductor 316, and the voltage across thetransformer windings is approximately zero. As is well know, thisadditional portion of the cycle permits the output voltage to be lessthan VB divided by the transformer's turns-ratio. How much less dependson the duty cycle. Since during normal operation the isolation stage isoperated at a fixed duty cycle in which power is always flowing frominput to output (except during the brief switch transitions), the valueof inductor 316 can be relatively small to achieve an acceptable outputripple. This reduces the size, cost, and power dissipation of thisinductor compared to what it might have been. During those times whenthe isolation stage is operated under a variable duty cycle, the ripplein the inductor current may then become large, but the larger outputvoltage ripple that results can usually be tolerated for start-up andshort-circuit conditions.

As mentioned above, during the freewheeling part of the cycle the diodesare carrying the inductor current. This is because the gate drive schemeshown in FIG. 3 would cause the MOSFET synchronous rectifiers to be offduring this part of the cycle. The additional power dissipation thatoccurs due to the higher on-state voltage of the diodes compared to thatof the MOSFETs can usually be tolerated for the start-up andshort-circuit conditions because they are normally short in duration.

If the output voltage is high, then it may be desirable to use acapacitive divider technique described in the '417 patent to reduce thevoltages applied to the gates of the MOSFET synchronous rectifiers belowthat of the voltages appearing at Nodes A and B. FIGS. 4A-4C show acircuit schematic of a product based, in part, on the ideas presentedhere and in the '417 patent. The function of the product is to provideisolation and a transformation of the input voltage to the outputvoltage according to the turns-ratio of the transformer. It does not, inits normal state of operation, provide regulation. As such it is a veryefficient product. One example of its use is to convert a 48V input to a12V output by using a turns-ratio of 4:1. Since there is no regulation,if the input voltage varies +/−10%, so too will the output voltage vary+/−10%. In certain applications, this variation in the output isacceptable, and well worth the very high efficiency of the converter,which is 96% in this example.

In addition, since the converter of FIG. 4 does not provide regulation,its output voltage demonstrates a droop characteristic. By this it ismeant that for any given input voltage, the output voltage dropsslightly as the output current increases. For instance, the outputvoltage may drop 5% as the output current varies from 0% to 100% of therated maximum value. This droop characteristic provides automaticcurrent sharing between two or more such converters that might be placein parallel.

Note in this schematic that the IC labeled U100 is a pulse widthmodulator (PWM) control chip that is normally operated such that thegate drive signals that pass through gate drivers U101 and U105 give thefixed duty cycle operation of the full-bridge described above. If thecurrent sensing amplifier U104-A senses that the current flowing on theprimary side of the circuit exceeds a threshold value, it commands thePWM control chip to reduce its duty cycle by an amount determined by howlarge the current gets above the threshold value. This provides acurrent limiting scheme for the product that protects against ashort-circuit condition.

Note also that comparator U106-A senses the duty cycle output of the PWMcontrol chip, and compares it to a threshold. If the duty cycle fallsbelow this threshold value, the output of the comparator causes the PWMcontrol IC to shut down. The circuitry around this comparator, includingtransistors Q111 and Q114, provides a latching mechanism such that thePWM control IC remains off once this condition is observed.

As described in the '417 patent and illustrated in FIG. 5, in somesituations, it may be desirable to place the isolation stage first inthe power flow, and to have the regulation stage follow. For example,when there are many outputs sharing the total power, the circuit mightbe configured as one isolation/step-down (or step-up) stage 501 followedby several DC-DC switching or linear regulators 503.

The DC power source to the full bridge primary circuit may provide avoltage that varies over the range of 36-75 volts. The output of theisolation stage may be 12 volts, and the regulation stage output may be5 volts or less. In particular, the regulation stage output may be 3.3volts. Typically, the regulation stage output is of a voltage level todrive logic circuitry.

Because the isolation stage uses synchronous rectifiers, it is possiblefor the current to flow from the output back to the input if, for agiven input voltage and duty cycle, the output voltage is too high. Thiscondition might, for example, occur during start-up where the duty cycleis slowly raised from its minimum value to its maximum value, but theoutput capacitor is already pre-charged to a high voltage, perhapsbecause it had not fully discharged from a previous on-state condition.It might also occur when the input voltage suddenly decreases while theoutput voltage remains high due to the capacitors connected to thisnode.

The negative current that results could cause destructive behavior inthe converter or in the system if it is not kept small enough.

One way to avoid this condition is to turn off either just the top twoprimary-side MOSFETs 101 and 104, or just the bottom two primary-sideMOSFETs 102 and 103, during the freewheeling period, as described above.By leaving the other two primary-side MOSFETs on, the voltage across theprimary and secondary windings of the transformer is guaranteed to beessentially zero during the freewheeling period. Given the gate drivescheme shown in FIG. 3, this, in turn, ensures the controlled rectifierswill be off during this part of the cycle.

With the controlled rectifiers off, negative current cannot flow duringthe freewheeling period. Negative current can flow during thenon-freewheeling part of the cycle, but since it must always start atzero, its value is limited to the ripple that the inductor permits,which is typically small enough to not cause a problem. This negativecurrent will be reset to zero at the start of each freewheeling period,either by providing a clamp circuit, as shown in FIG. 4D, or by allowingthe controlled rectifiers to avalanche and act as their own clamp. Sincethe clamp circuit must only work for a short duration, it need notrecover its absorbed energy and so can be simple, such as the one shownin FIG. 4D.

To limit the negative current, the isolation stage could operate in areduced duty-cycle mode. While the control circuit is typically designedto achieve this mode during start-up and shutdown of the isolationstage, it is not the normal mode of operation. If, during normaloperation, the input voltage drops suddenly, a large negative currentcan flow because there are no freewheeling periods.

To avoid this condition, the current flowing through the converter canbe sensed, either by sensing the load current directly, or by sensing asignal indicative of the load current. When the load current falls belowsome threshold, the duty cycle of the isolation stage can be reducedfrom its maximum value to provide freewheeling periods. Given the drivescheme for the primary-side MOSFETs outlined above, the negative currentwill then be kept small since the controlled rectifiers will be turnedoff for a portion of the cycle.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. For example, whereas the Figuresshow the secondary side rectification circuit arranged in a centertapped configuration with two secondary windings and two synchronousrectifiers, as is well known it could be a full wave rectificationconfiguration. One could use a full-bridge rectification circuit inwhich there is only one secondary winding and four synchronousrectifiers. Such a circuit reduces voltage stress on the synchronousrectifiers when they are off by a factor of two during normal operationof the converter.

1. A power converter system comprising: a normally non-regulatingisolation stage comprising: a primary winding circuit; a secondarywinding circuit coupled to the primary winding circuit, the secondarywinding circuit comprising a secondary transformer winding in serieswith a controlled rectifier having a parallel uncontrolled rectifier;and a control circuit which controls duty cycle of the primary windingcircuit, the duty cycle causing substantially uninterrupted flow ofpower through the primary and secondary winding circuits during normaloperation; and a plurality of non-isolating regulation stages, eachreceiving the output of the isolation stage and regulating a regulationstage output. 2-48. (canceled)